Add -r|--remove option to force abuild to remove the output directory

Add -r|--remove option to force abuild to remove the output directory
after every board build, in order to save disk space if you don't need
the actual output files.

Signed-off-by: Uwe Hermann <uwe@hermann-uwe.de>
Acked-by: Peter Stuge <peter@stuge.se>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@4119 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1